The family of Moors murders victim Keith Bennett have vowed to continue their fight to find his body following the death of his mother, who had led a near 50-year campaign for him to receive a Christian burial.

Winnie Johnson, 78, died in a hospice in the early hours of Saturday morning following a battle with cancer.

Her son's killer, Ian Brady, has repeatedly refused to say where he buried the 12-year-old boy on Saddleworth moor above Manchester, despite years of pleading from Mrs Johnson. On his website, searchingforkeith.com, Keith's brother, Alan Bennett, said on behalf of the family: "Winnie Johnson, the mother of Keith Bennett, died this morning at 12.35am, aged 78. She was a much loved mother, grandmother and great-grandmother, and is survived by one younger brother. Winnie fought tirelessly for decades to find Keith and give him a Christian burial. Although this was not possible during her lifetime, we, her family, intend to continue this fight now for her and for Keith. We hope that the authorities and the public will support us in this."

Mrs Johnson died unaware of claims that Brady had left details of the location of her son's remains in a letter to be opened on his death. The claims are featured in a Channel 4 documentary that is due to be broadcast on Monday. There were concerns the broadcast could be cancelled in light of Mrs Johnson's death. However, Channel 4 has confirmed the documentary will go ahead as planned following consultation with her family who have given their approval.

Brady, 74, and his partner, Myra Hindley, who died in jail in 2002 aged 60, murdered five youngsters in the 1960s. Keith's body is the only one that has not been found.

Mrs Johnson's lawyer, John Ainley, said that he believed Brady could still hold the key to finding the place where her son was buried. "Despite her personal appeals directly to Brady and via my office, Brady had persistently ignored the wishes of a grieving mother," he said. "She has died without knowing Keith's whereabouts and without the opportunity to finally put him at rest in a decent grave. It is a truly heartbreaking situation that this opportunity has now been irrevocably lost."

Mrs Johnson's death came as police investigated the possible existence of a letter in which Brady apparently discloses the location of Keith's body. Brady's mental health advocate, Jackie Powell, told the Cutting Edge documentary that the Moors murderer gave her a sealed envelope containing a letter to pass to Mrs Johnson in the event of his death.

It is understood Powell claims she returned the sealed envelope to Brady before her arrest. But there are suspicions the letter could be a cruel hoax perpetrated by Brady.

Detectives examining a large number of documents have so far found no letter or any evidence to suggest Brady has disclosed the location of Keith's body. "We are continuing to examine the documentation we seized to establish whether or not this letter even exists," said Martin Bottomley, head of investigative review at Greater Manchester police's major and cold case crime unit.

"Notwithstanding that, there is only one person who knows where Keith is buried and that is Ian Brady himself," Bottomley added. "I would implore him at this extremely sad time, and knowing that Winnie has died not knowing where Keith is buried, to at last do the decent thing and tell us where he is. Sadly, almost all of her life, she has had to live with the knowledge that Ian Brady refused to show compassion and do the right thing and disclose where he had buried her little boy."

Brady was due to go before a mental health tribunal last month, but it was delayed after he suffered a seizure. The tribunal was to consider his application that he be transferred to a prison and allowed to die. He has been fed by tube since he began refusing food 12 years ago.

David Kirwan, a solicitor who once represented Mrs Johnson and twice met Brady to discuss the possibility of him being taken back to the moors to help the search, described her death as "an absolute tragedy".

"I spent approximately four years helping Winnie in her campaign but she spent 48 years searching for her son," Kirwan said. "I would sum her up with one word: a mother."

Channel 4 executives met on Saturday eveningto discuss whether the Cutting Edge documentary, entitled Ian Brady: End-games of a Psychopath, should be broadcast as planned. It is understood a final decision on whether to go ahead with the broadcast will be taken only following a consultation with Mrs Johnson's family.